In a complex hydronic system, for instance a building heating system, hot water is pumped from a central boiler up a common riser from which it flows through a multiplicity of branch lines each including one or more terminals. Then the multiple streams are reunited in a common downpipe that leads back to the boiler. In such a system it is necessary to balance the flow in the individual branches. Thus each branch is provided with a balancing valve, which is nothing more than a lockable flow-control valve, that is adjusted until a predetermined flow, normally measured in gallons per minute, is obtained in the branch. In this manner a branch with particularly low resistance to flow does not get too much flow while another with relatively high resistance gets too little.
The standard method for adjusting flow in a branch, typical a radiator or coil, is fairly laborious. A typical-balancing valve (the Bell & Gossett ITT "Circuit Setter Plus," or the "CBV" system of Armstrong Pumps) has a graduated spindle that indicates the valve's flow cross section and its resistance to flow. Immediately upstream and downstream of the valve element of such a valve, the valve housing is provided with closable nipples that open internally into the valve. A differential pressure meter is first connected to these nipples. Then the balancing valve is adjusted to one of a multiplicity of graduated settings and the difference in pressure between the measuring locations upstream and downstream of the valve and the valve setting is noted. These readings are checked on a chart for the particular valve size to ascertain the flow rate in volume per units of time. The chart plots flow against differential pressure and has a large number of lines corresponding to the different settings so that the user checks the line for the setting being used, and determines for the pressure reading the flow. If the ascertained flow rate is not correct the valve is readjusted, a reading is taken again, and the appropriate line on the chart is consulted to get the new flow rate. Thus the balancing valve serves both for balancing and measuring flow. This process is repeated until the desired flow rate is obtained, and the valve is secured in the last position to lock in this flow rate, and a stop is set so that if the balancing valve is subsequently shut, for example for servicing of the associated radiator or chiller, it can be returned to the desired position, typically called the "memory" position.
Obviously this is an extremely laborious procedure involving making two separate connections to the line, and then adjusting the valve while frequently consulting a chart. The person doing this flow balancing must be fairly skilled, and even so the procedure can be very time consuming in a large building that can have hundreds of branches.
Another problem with the known system is that it is relatively inaccurate. The pressure-differential meter must normally be able to read pressure differentials over a relatively wide range. Like all such analog equipment, however, it is typically fairly accurate at one part, typically the high end, of its range but much less so toward the low end. Furthermore the valve positions must be very carefully monitored and the valves must be extremely accurately manufactured to ensure some close correspondence between the readings and the actual values. Thus if the flow in a line is relatively low the technician can be dealing with a combined inaccuracy of 20% or more.
Another problem with the known systems is that connecting the differential pressure meter to the branch is a laborious problem, entailing making two separate connections at two separate locations. There is frequently some leakage, and the taps provided for the differential pressure meter are themselves subject to leakage or vandalism.